The secret sauce is the plain text file named persdict.dat in your Firefox profile directory. Despite its name, that file contains a human-readable list of words you've added to your dictionary. If you work across several machines with Firefox and don't want to add custom words to the dictionary on each machine, you can move this file between machines or simply copy and paste lists of words (like tech terms or company names that you use often) into this file. Big ups to Mozilla for making Firefox's internal configuration files human-readable and editable outside of its interface, unlike, ahem, some other software companies (whose name appears in the included screenshot.)